Chosen Solution
Device : Xiaomi Redmi 3 pro After replacing the battery, the mobile network doesn’t work when back cover is properly clipped back on. After testing, it works fine without the back cover and absolutely not when back cover is clipped back on. I double checked all the internal connectors (one was bent so I straightened it back) and tape antenna. I tried loosening or tightening screws. I tried to replace the old battery (matter of the 1mm difference) without success. I lack magnifying glass so not easy to check if a component is damaged (but why would it work without the back cover?).
It sounds like something is blocking or causing interfarance . Try removing all the screens n put the black cover on to see if that fixes it. If it does put one screw in at a time then check to see if it still working or not .
